Use qDeleteAll in destructor of Scene class
Summary:
qDeleteAll can be used to delete values in QMap or QHash. Please notice
that qDeleteAll calls delete only for values, not keys.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D13822